Deduplicate "is function type A a subtype of function type B"

Filing an issue for https://github.com/bytecodealliance/wasm-tools/pull/2478#discussion_r3034057368

We currently have two subtype routines:

* One in [`matches.rs`](https://github.com/bytecodealliance/wasm-tools/blob/783c35724a8908e2894399ecce1f9ee064710134/crates/wasmparser/src/readers/core/types/matches.rs#L142-L208)
* Another in [`operators.rs`](https://github.com/bytecodealliance/wasm-tools/blob/783c35724a8908e2894399ecce1f9ee064710134/crates/wasmparser/src/validator/operators.rs#L1942-L1960)

They're a bit subtly different insofar as one has `WithRecGroup` and the other doesn't. I'm not entirely sure how to unify these, but it would be good to.